1. Field of the invention
The present invention relates to a semiconductor photoelectric device, and more specifically to an improved semiconductor photoelectric device utilizing a semiconductor composite comprising a tin oxide film deposited on a semiconductor substrate and having a rectifying characteristic.
2. Description of the Prior Art
One of the typical existing photoelectric devices made of semiconductor materials is a silicon photoelectric device. As is well known, the silicon photoelectric device is manufactured by forming a P-type diffused layer or a thickness of a few microns or less on the surface of an N-type silicon substrate so that when the light impinges on the P-N junction formed therebetween, photovoltaic energy is generated between the P-type layer and the N-type layer. Also it is well known that the conductivity type of the respective layers may be reversed depending upon the application.
However, this type of silicon photoelectric device is disadvantageous in that the device is expensive as compared with other types of photoelectric devices, such as cadmium sulfide photoelectric devices, mainly because manufacture of the silicon photoelectric devices necessitates a diffusion process which should be carried out at a high temperature and under a delicately controlled condition. Another disadvantage of a silicon photoelectric device is that it is of reduced sensitivity in the short wavelength region of its spectral characteristics due to the fact that light energy greater than the forbidden band of energy of the semiconductor does not transmit through the semiconductor to the P-N junction, which usually is formed at a certain distance from the surface of the semiconductor. Thus, it is essential to make the abovementioned diffused layer extremely thin, preferably as thin as 0.3 micron in order to implement such a device which is sufficiently sensitive to the shorter wavelength region as well. Nevertheless, formation of a thin diffused layer calls for a high level of diffusion techniques, again resulting in a high cost of this type of device.
If the diffused layer of such silicon photoelectric device could be replaced with a transparent conductive film or metal oxide and if such film could serve the same function as that of the diffused layer, the cost of manufacturing a photoelectric device would be greatly reduced and the resultant device could have a greater scope of application. Of interest in this connection is U.S. Pat. No. 3,679,949, entitled "SEMICONDUCTOR HAVING TIN OXIDE LAYER AND SUBSTRATE", issued July 25, 1972 to Genzo Uekusa et al. and assigned to the same assignee of the present invention. The referenced patent basically discloses a semiconductor composite comprising a film of tin oxide (SnO.sub.2) deposited on a semiconductor substrate such as silicon and having rectifying and photoelectric characteristics therebetween.
More specifically, the referenced patent discloses such composite obtained by a process comprising the steps of heating an N-type silicon single crystal substrate in a quartz tube, and introducing a vapor of a tin salt such as dimethyl tin dichloride ((CH.sub.3).sub.2 SnCl.sub.2) into said quartz tube for depositing a tin oxide film on said silicon substrate by pyrolysis. Such composite comprises a barrier formed between the tin oxide film and the silicon substrate, which barrier is presumably a Schottky barrier and closely resembles a P-N junction in a rectifying characteristic. Such barrier may be advantageously utilized as a rectifying device or photoelectromotive force device. As is well known, the tin oxide film is transparent and conductive. Hence, by so adapting the composite that the light is applied to said barrier through the tin oxide film, a photoelectric device is provided. The spectral characteristic of such photoelectric device is such that it is more highly sensitive in the visible wavelength region as compared with a conventional silicon photoelectric device. It also exhibits a higher output at lower illumination, and is satisfactory in temperature characteristic and response characteristic. Another advantage of the referenced patent composite is that the composite can be provided with ease and less cost on a mass production basis in view of the fact that the tin oxide layer may be deposited at a lower temperature as compared with a process employed in manufacture of the silicon photoelectric device.
Preferably silicon is employed as a semiconductor substrate material in manufacturing the referenced patent composite. It should be pointed out, however, that the surface of the silicon substrate is likely to be oxidized even at a normal temperature and as a result the silicon substrate as prepared for manufacture of semiconductor devices usually comprises a thin oxide film formed on the surface thereof. Such oxide film typically comprises SiO.sub.2. Again it should be pointed out that an additional oxide film is formed on the surface of the substrate in the course of further depositing a tin oxide film on the surface. As a result it was found that the semiconductor composite prepared in accordance with the teaching in the said referenced patent usually comprises a very thin insulating film, typically of SiO.sub.2, of a thickness of a few A. to approximately 10A. incidentally formed between the tin oxide film and the substrate. Thus it would be readily understood that such undesired intervening layer of insulating film is inevitably formed, unless consideration is given to eliminate such undesired layer.
With a view to investigating in detail what influence the SiO.sub.2 layer incidentally formed between the SnO.sub.2 layer and the Si substrate has upon performance of the SnO.sub.2 --Si heterojunction of the composite, an experiment was carried out, in which removal was first made of the SiO.sub.2 layer formed on the substrate surface through natural oxidization of the substrate material and then deposition was made of an SnO.sub.2 layer on the fresh surface of the substrate by a process and means for eliminating formation of an SiO.sub.2 layer on the substrate surface during deposition of the SnO.sub.2 layer, so that a new composite was provided, which comprises no substantial SiO.sub.2 layer between the SnO.sub.2 layer and the substrate of the composite. It was observed that the resultant SnO.sub.2 -Si composites lack uniformity in reverse breakdown voltage, and have an increased reverse leakage current and a lowered reverse breakdown voltage. As is well known to those skilled in the art, these changes in characteristics with respect to the referenced patent composite are all disadvantageous in various applications of the semiconductor device. Thus the fact was confirmed that formation of the SiO.sub.2 film at a junction region of the the SnO.sub.2 -Si composite as a significant influence upon the characteristics of the semiconductor device.
Nevertheless, the fact was also confirmed by experiment that the thickness of the SiO.sub.2 film incidentally formed in the SnO.sub.2 --Si composite manufactured in accordance with the teaching in the referenced patent does not exceed 15A. It is believed that usually such a very thin SiO.sub.2 layer does not cover the whole surface of the silicon substrate; or instead the substrate surface is studded with a plurality of small SiO.sub.2 film areas with irregularities of the film thickness and other film conditions. For this reason it is extremely difficult to provide SnO.sub.2 --Si composites of uniform in characteristics as a semiconductor device, resulting in an unsatisfactory yield rate of manufacture of the device.
Another semiconductor photoelectric device of interest, which has a different structural feature, is disclosed in U.S. patent application, Ser. No. 304,809, entitled "SEMICONDUCTOR DEVICE", filed Nov. 8, 1973, by Shigeru Tanimura et al. and assigned to the same assignee of the present invention. Briefly stated, the referenced application discloses a semiconductor composite comprising a semiconductor substrate, an insulating film formed on said semiconductor substrate and a film of a tin oxide, preferably stannic oxide (SnO.sub.2), deposited on said insulating film and having a rectifying characteristic. Preferably the material of said semiconductor substrate is selected from the group consisting of Si, Ge and GaAs. Preferably the material of said insulating film is selected from the group consisting of SiO.sub.2, Si.sub.3 N.sub.4 and GeO.sub.2. A thickness of the insulating film is chosen to be in the range of 15A. to 500A, but preferably a thickness of the insulating film is chosen to be in the range of 27A. to 300A. and more preferably 27A. to 100A.
Such a composite can be used as an ordinary rectifier without any incidental radiation energy and, with its SnO.sub.2 layer as a light receiving side, can be used as a photoelectric device. However, other applications of the referenced application are a voltage controlled switching device ad a light controlled switching device. More specifically, it was discovered that the composite, as subjected to radiation energy of a certain value, if a thickness of the SiO.sub.2 film is chosen to be a particular value range, say 2A. to 500A., shows an excellent voltage response to a reverse bias voltage applied to the composite. Thus, the composite as subjected to a predetermined value of radiation energy may be used as a switching device which is operable as a function of the voltage applied to the composite in a reverse direction. It was also discovered that the composite as supplied with a certain value of a reverse bias voltage or with no bias, if a thickness of the SiO.sub.2 film is chosen to be a particular value range, say 27A. to 500A., shows an excellent radiation response in a reverse current to radiation energy applied to the composite. Thus, the composite may be used as a switching device which is operable as a function of the radiation energy applied to the composite.
One of the most advantageous features of the devices disclosed in the referenced patent and the referenced patent application as compared with the conventional silicon photoelectric devices is that the referenced photoelectric device comprising an SnO.sub.2 --Si composite or SnO.sub.2 --SiO.sub.2 --Si composite shows a remarkably improved photosensitivity even at low illumination, while such photoelectric device can also respond to illumination higher than the limit of the conventional silicon P-N junction device. More specifically, the referenced photoelectric device can respond to illumination of a range from as low as 10.sup.-.sup.3 lux to as 100,000 lux, while the conventional silicon P-N junction device can only cover an illumination range from 10.sup.-.sup.2 lux to 30,000 lux at the best. In particular, good use can be made of the high sensitivity response at low illumination levels of the referenced device.
Typical requirements of a semiconductor photoelectric device are such that a dark current is small in consideration of using the device at low illumination and that an open voltage of the device as subjected to radiation energy is high. It is well known that generally a dark current of a semiconductor diode is closely related to a reverse saturation current and thus the dark current is reduced by making small the reverse saturation current. This is understood by the following equation, which expresses a rectifying characteristic. EQU Id = Io (1 - e.sup.-.sup.qV/nkT)
where Id is a dark current, Io is a reverse saturation current, V is a reverse bias voltage, q is an elementary electric charge, n is a junction coefficient, k is a Boltsman constant, and T is a temperature of the device.
On the other hand, it is also known that the reverse saturation current Io is related to a barrier height of the junction of the device. Generally, the higher the height of the junction barrier is, the smaller the reverse saturation current Io becomes. This relation is expressed by the following equation. EQU Io = q .sup.. Vth .sup.. N .sup.. e.sup.-.sup.gV D.sup./nkT
where N is carrier density in a silicon substrate, Vth is velocity of the carriers caused by heat energy, and V.sub.D is a height of the junction barrier.
It has further been known that photoelectromotive force or an open voltage becomes higher as the reverse saturation current is made small. This relation is expressed by the following equation. ##EQU1## where Vop is an open voltage and I.sub.L is a light current.
In view of the above discussion, it is seen that it is desired to provide an improved photoelectric device of the referenced prior art type wherein a reverse saturation current is made small.
In some applications of the referenced prior art device, it is often required to provide a device capable of generating an output current of a sufficient value. For this purpose, those skilled in the art might simply think of implementing a composite of an increased light receiving barrier area. As a result of experimentation, however, it was found that in the referenced prior art composite an increase of the barrier area in a single light receiving region does not bring a linearly proportional reverse dark current but rather causes a much more increased reverse dark current than expected. It was also found that a light current in terms of per square of the composite subjected to illumination is decreased, possibly because of an increased series resistance across the increased barrier area. Thus it was observed that a signal-noise ratio of the referenced prior art composite is degraded, as the barrier area for light receipt in a single region is increased. Such a disadvantage is particularly aggravated in utilizing the composite at low illumination.